One similarity between land based empires between 1450-1850 CE was the use of gunpowder. Gunpowder was used to create strong armies as a unified state. The use of gunpowder lead to a massive amount of power like the ottoman taking over Byzantine and fighting other forces. altjogunt gunpowder is just one example, you could also use the use of religion in each empire, bureaucracy used, tax farmers, etc. !!